The repeaters mounted aboard known communication satellites comprise emission and reception systems which rely on transparent architectures offering uplinks between at least one terrestrial station and the repeater and downlinks between the repeater and a plurality of users. The emission and reception systems can cover a terrestrial geographical zone of coverage by means of a lone beam forming a single ground spot or by means of several beams forming multiple ground spots.
FIGS. 1a and 1b show two examples of architecture of a contemporary payload emission and reception system offering uplinks and downlinks, respectively with single-spot coverage and with multi-spot coverage. The repeater comprises a signal receiver 10 intended to receive radiofrequency signals 40 originating from an uplink, such as for example a terrestrial station. The radiofrequency signals emitted by a terrestrial station generally occupy a broad frequency band. In the signal receiver 10, the broadband signals 40 received are frequency transposed by a frequency converter so as to pass from the reception frequency band to the emission frequency band dedicated to the users. The frequency converter is controlled by a local oscillator OL, the local oscillator being able to operate at a predetermined fixed frequency or at a frequency modifiable by remote control. After frequency conversion, the frequency-transposed broadband signal is thereafter transmitted to an input demultiplexer IMUX 11 which splits and filters the broadband signal into several different frequency sub-bands. Each frequency sub-band, also called a channel, is thereafter amplified by a dedicated power amplifier 12 before being retransmitted to the terrestrial users. In an architecture with single-spot coverage such as represented in the example of FIG. 1a, the signals conveyed in the various channels 1, 2, 3, 4 are recombined in an output multiplexer OMUX 13 to form a single pathway before being emitted towards the users, by a single emission antenna source, not represented. In a multi-spot architecture such as represented in FIG. 1b, the signals conveyed in the various channels are emitted in the form of mutually independent beams by emission sources 14 of an emission antenna, the signals emitted in two adjacent beams 15, 16 being able to recombine, in an overlap zone, not represented, while they are being radiated. The emission antenna which ensures the multibeam coverage can be of the one source per beam type or of the multi-source per beam type. In the case of an antenna of multi-source per beam type, a beamforming array is used.
These two architectures exhibit a significant problem due to the fact that the filters 9 of the input demultiplexer 11 which ensure the filtering of the various frequency sub-bands 1 to 4 are not perfect and allow through, in each channel, spurious frequencies belonging to the sub-bands allotted to the adjacent channels. Each channel therefore transports a main signal 18 whose frequency lies in the frequency sub-band dedicated to the corresponding channel and spurious signals 19 whose frequencies are situated in the adjacent frequency sub-bands. The various channels then form various possible paths for the signals received in the various filtered frequency sub-bands. The signals of the same frequency that are conveyed in two different adjacent transponders and which have therefore followed different paths have different amplitude and phase levels but are mutually coherent. During recombination of the signals arising from the various channels, in the output multiplexer or by radiation, the main signals and the spurious signals of the same frequency arising from adjacent channels then recombine with one another in a coherent manner, thereby modifying the gain and the variation of the delay law and impairing the quality of the link. The contribution of the spurious signals, that have followed different paths from the main path, to the amplitude and to the phase of the radiated signal are conventionally called multi-path effects.
To solve this problem, in the case where the signals are recombined by an output multiplexer, the frequency bands of two adjacent filters are generally separated by a guard band, thereby making it possible to guarantee good frequency isolation between two adjacent channels and to decrease the effects of the multi-paths. However, this solution exhibits the drawback of decreasing the width of the frequency band that can be used for the signals to be transmitted.
In the case of multi-spot coverage where the signals of the same frequency can recombine by radiation, the beams radiated by the emission sources of each channel are generally isolated spatially from one another to ensure sufficient spatial isolation between two beams. This solution imposes significant frequency allocation constraints on the adjacent beams and is very constraining in respect of the dimensioning of the repeater.